Siccative organic coating compositions have been applied to metal surfaces such as by spraying, dipping, rolling, centrifuged dip-spinning, etc. In recent years, various water-soluble resin-based paints and lacquers have been developed, and progress has been made toward the application of such coating systems by electrophoresis. The electrophoretic application of paint and lacquer involves the phenomena of electro-osmosis and electrolysis, as well as electrophoresis. In this method, an electric current is passed through the paint or lacquer solution while the article to be painted is made an electrode, usually the anode, in the paint or lacquer.
The electrodeposition of water-based coatings has been employed to process metal parts including small stamped parts such as nuts, bolts, and fasteners. The use of electrodeposition of siccative organic coatings on small parts has advantages over other methods of coating. For example, the process deposits a film of uniform thickness on essentially any conductive surface, even those which have sharp points and edges. The electrocoated film when applied, is relatively water-free and, thus, will not run or drip when taken out of the bath. The use of water-base coating compositions also is advantageous since they contain little or no organic solvents or other volatile organic compounds. Accordingly, such aqueous systems and processes do not require special precautions or equipment for handling any harmful volatile materials, and such aqueous systems and processes do not contribute to the problem of volatile organic emissions and air pollution.
It also is well known in the metal-finishing art that metal surfaces such as aluminum, ferrous and zinc surfaces may be provided with an inorganic phosphate coating by contacting the surfaces with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of a siccative organic coating composition such as paint, lacquer, varnish, primer, synthetic resin, enamel, and the like. Procedures also have been described in the art for improving the rust-resistance of metal articles by the application of a film of paint over phosphated surfaces. Although the application of a siccative coating over a phosphated metal surface improves the corrosion resistance properties of the metal, there continues to be a need to improve the corrosion resistance of electrophoretically painted metal surfaces.
Procedures for improving the rust resistance of metal articles by application of a film of paint over a phosphated surface have been described in a number of patents such as U.S. Pat. Nos. 3,454,483; 3,620,949; 3,864,230; 4,007,102; 4,165,242 and Re 27,896. As noted in U.S. Pat. No. Re 27,896, the electrophoretic application of paint and lacquer over a phosphated metal surface is not a complete solution to the rust problem. It has been found that when paint is electrodeposited on phosphate coated ferrous metal surfaces, the resulting paint films have often been found to contain numerous small depressions or pin holes. Such films generally provide only a slight corrosion protective action, probably due to the presence of a lower film thickness in the depressions. In an attempt to overcome this problem, paints and lacquers have been used containing synthetic resin components which form films which during a subsequent baking will soften so that the surface blends smoothly and the depressions and pin holes are filled with resin. One difficulty with this technique, however, is that there often is a withdrawal of the paint film from the edges of the workpiece being treated so that these portions are then subjected to additional corrosion attach. In U.S. Pat. No. Re 27,896, a solution to this problem is suggested which involves the incorporation of cupric ions into the zinc phosphate coating applied to the article prior to painting. In U.S. Pat. No. 3,454,483, an improvement in the corrosion resistance of electrophoretically painted metal surfaces is suggested when the phosphate coating used as a primer for the paint contains fluoride ions.
U.S. Pat. No. 4,165,242 describes a method for treating metal parts either singly or in bulk barrel processing to provide durable and rust-inhibiting coatings which comprises the steps of
(a) treating the metal parts with an aqueous phosphating solution to deposit a phosphate coating thereon, PA1 (b) electrophoretically depositing a siccative organic coating on the phosphate coated metal parts, and PA1 (c) treating the siccative organic coated part with an oil to deposit a corrosion-inhibiting top coat. PA1 (A) phosphating the metal parts with an aqueous metal phosphating solution; PA1 (B) electrophoretically depositing a film of siccative organic coating composition on the phosphated metal parts; PA1 (C) applying a second film of a coating composition comprising at least one film-forming organic resin component as a seal coat; PA1 (D) curing the coating on the metal parts; and optionally PA1 (E) applying a corrosion-inhibiting film as a top seal coat.